1. Field of the Invention
This invention relates to method and apparatus for the manufacture of a laminate of two metal sheets and a layer of thermoplastics material sandwiched between them and adhering to them, particularly the continuous manufacture of such a laminate in which the two sheets are moved along paths to a convergence region and the thermoplastics material is continuously extruded. In the convergence region, the thermoplastics material is squeezed between the sheets to form the layer of the laminate. The invention also relates to the presence of an additive in such a laminate, which provides increased durability, at least against attack by water.
2. Description of the Prior Art
GB-A-2038239 describes a method of forming a laminate of the type indicated above. The pressing together of the first and second metal layers may be carried out for a period of from a few seconds up to about 30 minutes. The temperature of the laminate during pressing is maintained in the range from 150.degree. C. to 329.degree. C. (Example 19 to 21 and 155 respectively).
There is at present a desire to improve the bonding strength and durability of such a laminate, particularly in order to provide new lightweight sheet materials. A possible use for such a material is as a generally horizontal or generally vertical exterior part of a motor vehicle, for which purpose especially high requirements are placed on the quality of the bond between the thermoplastics material of the laminate an the metal sheets. As well as the quality of the bond, i.e. its strength particularly peel strength, the durability of the bonding is also of concern where there is continual penetration of water and also temperature fluctuations. In addition, such a material must be able to undergo deformation processes, in order to achieve the desired shape, and such deformation processes may be cold or hot. Meanwhile, the laminate must retain its strength and durability.
Furthermore it is desirable that the additional step and expense of providing adhesive layers between the extruded thermoplastic material and the metal sheets is avoided.
EP-A-87546 describes a method in which an extruded polymer band, two adhesive layer sheets and the two metal sheets are brought to a nip between a pair of rollers, and squeezed to effect thickness reduction of the polymer band, prior to cooling of the laminate. The thickness reduction may be in the range of 10 to 90%. GB-A-1277350 also shows thickness reduction of the thermoplastics layer during formation of the laminate. Adhesive layer sheets are again employed.
GB-A-1092715 describes a method of forming or shaping aluminium-clad thermoplastic laminates with the application of heating. Acrylonitrile-butadiene-styrene copolymer (ABS) is mentioned as one possible material GB-A-1014154 describes the formation of such laminates and their cold forming. In an example, hot extruded polyethylene is fed into the nip of cooperating pressure rolls together with aluminium foils and is bonded directly to the aluminium.